Within various well known fluid dynamic operating systems there exists various staged valving systems, which valving systems often may comprise a first stage or pilot valve configured to provide an output to a subsequent second stage valve or valving system present downstream from the pilot valve. The output from the pilot valve is typically a fluid control pressure that is proportional to the input control signal. This output control may be utilized within the subsequent valve or valving system for one or more purposes, such as to dictate the operational performance of the second stage valve. For example, the control pressure may be utilized to operate a main or interim pressure control valve configured to control flow of fluid under pressure to various actuating components, such as a hydraulic actuator.
Electro-magnetically operated pilot valves for controlling a pressure in proportion to the modulation ratio of a pulse width modulated electrical signal, or in proportion to a voltage level applied to them, are well known. One type of conventional pilot valve comprises a valve spool movably mounted within a valve body for variably coupling the valve inlet port to the valve outlet port. A motor, such as an electrical rotor motor, is mounted on or within the valve body, and is responsive to electrical input control signals that actuate the motor to apply a variable pressure to one end of the valve spool. Valve outlet pressure is fed back to the opposing end of the valve spool. This pressure acts on the effective area of the valve spool, creating a force opposing the motor. Pilot valve outlet control pressure is therefore a function of the input force applied by the motor, which in turn is a function of magnitude of the input control signal applied to the motor.
One problem associated with conventional pilot valves utilizing a valve spool is that these are sensitive to movement of the valve spool, especially if scaled down to be operable within a micro environment. Another problem is that the lands of the valve spool are only capable of providing an abrupt change in area with respect to the distance displaced. In other words, the percentage of the diameter of the orifice or port that is opened determines the amount of flow. This can be expressed as the rate of change in the area of the orifice or port with respect to the rate of change of the displacement of the valve spool, which is the gain of the system. Conventional valves utilize a valve spool with sharp edged lands, which greatly increase the gain of the overall system since the rate of change is abrupt.
Another common type of pilot valve may be referred to as a flapper valve. A conventional flapper valve comprises a magnetic torque motor (utilizing a magnet, a coil, a magnetic plate, and magnetic pole pieces) configured to provide an input control signal to control movement of an armature, which in turn, produces movement in a separate flapper component coupled to the armature. The flapper is positioned between opposing nozzles having equal fluid flow with equal resistance. Pressurized supply fluid continuously flows through both inlet orifices, through the opposing nozzles, and through a drain orifice to the return. In response to the rocking motion of the armature, the flapper is caused to move to throttle fluid flow through one nozzle or the other, thus diverting flow to one of two ends of a valve spool. The spool slides in a sleeve or bore of a valve body that comprises ports that fluidly connect to the supply pressure and return. At null, the spool is centered in the valve body, just covering or closing the pressure and return openings. Movement of the spool to one side or the other allows fluid to flow from the pressure supply to one control port and from the other control port to the return. In doing this, a pressure differential is created that causes the valve spool to displace to open corresponding ports, thus providing a control pressure output.
The flapper valve further comprises a feedback system in the form of a spring coupled to the flapper that engages the spool. The spring is configured such that movement of the spool displaces the spring to create a restoring torque on the flapper, and thus the armature. As the feedback torque becomes equal to the torque from the motor, the armature and flapper are caused to move back to a centered position. Therefore, the position of the spool is proportional to the input signal to the motor. In addition, with constant pressures, flow to load is proportional to spool position.
There are several problems associated with conventional flapper valves, particularly if scaled down to be used in a micro environment. First, they have high quiescent losses. Indeed, when null and with the flapper at rest between the nozzles, the flapper valve has a propensity to leak a tremendous amount of fluid through the nozzles. This is true in a macro or micro environment. If an attempt is made to decrease the amount of leakage by reducing the size of the nozzle orifices, the result is a decrease in fluid flow, and therefore a decrease in bandwidth. Although the amount of leakage may be reduced, the output efficiency is decreased. In other words, larger scale valves may be less efficient, but they provide better output. Conversely, smaller scale valves, while perhaps more efficient, provide less output. In order to get the amount of fluid flow necessary to drive the valve spool at a high frequency, a certain size orifice is needed. However, with such a suitably sized orifice, when the system is at rest, the gap between the nozzles and the flapper is large and the system leaks fluid, thus making the valve inefficient. Second, scaling down conventional flapper valves to a size suitable for operation in a micro environment is both difficult and costly. Micro environments may require operating valves to be on the order of one hundred to several hundred microns. Machining the component parts and orifices to correspond to this size is cost prohibitive. Third, scaling down conventional flapper valves increases their sensitivity to valve spool displacement since the required distance to move the valve spool is significantly reduced. Fourth, scaled down flapper valves may be unstable under desired operating parameters. Indeed, the control pressure from the pilot valve has to be stable in order to properly service the next valve. This is especially true in the case of operating at high frequencies. If a conventional flapper valve is reduced in size too much, it has a greater chance of being perturbed as a result of the flow through orifices that are too small to handle the required capacity. Stated differently, if scaled down to perform in a micro environment, conventional flapper valves will perturb and react unreliably to downstream loads (loads acting against the control or output pressure of the pilot valve) because the corresponding orifices are not large enough to handle the flow of the fluid. Other problems may be recognized by those skilled in the art.